If deoxys is in-game or competitive, while leveling up(if not at 100)switch between the forms and get the different moves from the formes, move tutors, move relearner, whatever. Example:deoxys-D with extremespeed, a move from the deoxys-S moveset. For competitive, do the above and end in defense form, maxing out whatever stat. If just for in-game stuff, same thing, just any forme you want.